1. Field of the Invention
This invention relates to a thermosensitive image transfer ink sheet capable of producing images with excellent heat resistance, solvent resistance, and frictional resistance, suitable for printing bar code labels for use in FA (Factory Automation) which demands images with such properties. This invention also relates to a thermal image transfer recording material using the thermosensitive image transfer ink sheet in combination with an image receiving sheet.
2. Discussion of the Background
A conventional thermosensitive recording material has the shortcomings that printed images are vulnerable to the changes in the environmental conditions such as changes in ambient temperature and light, so that recorded information tends to be easily erased by the changes in such environmental conditions.
In sharp contrast to this, a thermosensitive image transfer ink sheet has the advantages over the conventional thermosensitive recording material that images can be printed on plain paper, printed images can be preserved for an extended period of time, and information documents prepared from the thermosensitive image transfer sheet by computers and word processors can be output at low costs and substantially noiselessly. Because of these fundamental features, a thermosensitive image transfer ink sheet has recently become rapidly popular.
In accordance with the development of FA, there is an active demand for a thermosensitive image transfer ink sheet provided not only with the above-mentioned fundamental features, but also with additional features such as excellent heat resistance, solvent resistance, and frictional resistance.
To meet this demand, it is required that a resin component in a thermofusible ink layer of a thermosensitive image transfer ink sheet be three dimensionally crosslinked. The simplest way for doing this may be, for example, crosslinking the entire resin components in a thermofusible ink layer before image transfer is carried out. However, in this method, a large amount of heat and time is required for crosslinking the entire resin components in the thermofusible ink layer, and when the temperature of the thermosensitive image transfer ink sheet is suddenly increased for such crosslinking, there is the risk that the thermosensitive image transfer ink sheet adheres to each other when rolled, that is, the so-called blocking occurs, and also there is the risk that many creases are formed on the surface of the thermosensitive image transfer sheet, which have adverse effects on the quality of printed images. Moreover, if the entire resin components in the thermofusible ink layer have already been crosslinked, a large amount of thermal printing energy is required to transfer the thermofusible ink to an image receiving sheet, so that it is difficult to increase the thermosensitivity of the ink sheet. Furthermore, it is difficult to perform instant image transfer by the application of a small amount of thermal energy to the ink sheet by use of a high-speed printer which is recently available.
In addition to the above, when the amount of energy applied to the thermosensitive transfer ink sheet is increased to obtain an image with a satisfactory density, there is the risk that hot wires in a thermal head are cut off while in use.
In order to eliminate the above-mentioned drawbacks, various studies have been made, which are directed to the achievement of instant crosslinking of resin components contained in a thermofusible ink layer of a thermosensitive image transfer ink sheet during image transfer.
These studies include, for instance, a method of containing a thermocrosslinking resin compound in a thermofusible ink layer of a thermosensitive image transfer ink sheet as disclosed in Japanese Laid-Open Patent Application 60-212389; a method of separating a thermosensitive ink layer into two layers, one layer containing a reactive polymeric compound, and the other layer containing a crosslinking agent, whereby the reactive polymeric compound is instantly three-dimensionally crosslinked by the thermal energy applied from a thermal head at image transfer as disclosed in Japanese Laid-Open Patent Application 63-254093; and a method of containing a photo-crosslinking initiator in a thermosensitive ink layer of an thermosensitive image transfer ink sheet, and instantly crosslinking resin components contained in the thermosensitive ink layer by the application of natural light or special light as disclosed in Japanese Laid-Open Patent Applications 60-132790 and 62-23784.
Additionally, there is a method of containing one of a crosslinking resin or a crosslinking agent in a thermofusible ink layer of a thermosensitive image transfer ink sheet, and containing the other in a surface portion of an image receiving sheet, and combining the thermosensitive image transfer ink sheet and the image receiving sheet when image transfer is carried out. This method is considered comparatively easier to separate the crosslinking resin from the crosslinking agent than the previously mentioned methods, so that a large number of studies have been made on this method.
A representative example of the above-mentioned method is disclosed in Japanese Laid-Open Patent Application 62-87389. In this method, a crosslinking agent such as isocyanate is contained in a surface portion of an image receiving sheet, and is crosslinked with one component of hot melt materials contained in a thermofusible ink layer of a thermosensitive image transfer ink sheet.
Another representative example of the above-mentioned method is disclosed in Japanese Laid-Open Patent Application 2-41289, in which a polyol serving as a hardening polymer and isocyanate serving as a curing agent are separated, one in a thermofusible ink layer of a thermosensitive image transfer ink sheet and the other in an image receiving sheet.
Furthermore, Japanese Laid-Open Patent Application 63-212588 discloses a method of crosslinking a resin component in an ink layer of a thermosensitive image transfer ink sheet by use of a specific image receiving sheet which contains a micro-capsuled isocyanate and a thermoplastic polyol in a surface portion thereof.
In the method described in the previously mentioned Japanese Laid-Open Patent Application 60-212389, the thermofusible ink layer of the thermosensitive image transfer ink sheet comprises (a) a thermocrosslinking resin, which is composed of a combination of a blocked isocyanate which is blocked by a blocking agent such as acetylacetone or phenol, and a polyvalent active hydrogen-containing compound such as polyester polyol, or (b) a thermocrosslinking resin compound composed of a combination of epoxy resin and a micro-capsuled amine. These thermocrosslinking resin compounds, however, have the shortcoming that the crosslinking reaction is very slow. In the thermocrosslinking resin compound which contains the micro-capsuled amine, the particle diameter of the micro-capsuled amine is in the range of about 5 to 10 .mu.m, so that the thermofusible ink layer tends to become significantly thick. In this case, there is also the shortcoming that the micro-capsules in the thermosensitive image transfer ink sheet are ruptured by the pressure applied thereto when the image transfer ink sheet is rolled for preservation, so that a crosslinking reaction is caused to take place in the thermosensitive image transfer ink sheet. The result is that the amount of thermal energy to be applied for thermal image transfer has to be increased.
In the method in the previously mentioned Japanese Laid-Open Patent Application 63-254093, a primary amine or an acid anhydride is employed as a crosslinking agent. In view of the reactivity of a primary amine, even when the crosslinking resin and the resin are separately contained in two ink layers of the thermosensitive image transfer ink sheet, respectively, it is difficult to consider that the resin and the crosslinking resin react at the interface between the two layers only during the thermal image transfer.
When an acid anhydride is employed, three dimensional crosslinking does not take place, but only linear crosslinking takes place, even if some crosslinking takes place, so that images with excellent heat resistance, solvent resistance, and frictional resistance cannot be obtained by the method disclosed in Japanese Laid-Open Patent Application 63-254093.
Furthermore, in the previously mentioned Japanese Laid-Open Patent Applications 60-132790 and 62-23784, the thermosensitive ink layer of the thermosensitive image transfer ink sheet contains a photo-crosslinking initiator. However, when natural light is employed, at least half a day will be required for crosslinking resin components contained in the thermosensitive ink layer. In order to perform the instant crosslinking after thermal transfer, a special printer provided with a special light source is indispensable. Therefore, this method is not suitable for general use.
In the method disclosed in the previously mentioned Japanese Laid-Open Patent Application 62-87389, a crosslinking agent for one component of hot melt materials in the thermosensitive image transfer ink layer is contained in a surface portion of an image receiving sheet. However, if a crosslinking agent such as a primary amine or acid anhydride is employed, in view of the reactivity thereof, such a crosslinking agent in the surface portion of the image receiving sheet will deteriorate during preservation for an extended period of time, so that the effect of the crosslinking agent will be significantly decreased when used in practice.
Furthermore, when a hardening polymer and a curing agent are merely separated, one in a thermofusible ink layer of a thermosensitive image transfer ink sheet and the other in an image receiving sheet, as described in Japanese Laid-Open Patent Application 2-41289, if the separation of the hardening polymer from the curing agent is satisfactory, the curing agent such as an isocyanate compound is extremely unstable, and easily reacts with water in air so that the curing agent itself is crosslinked. Therefore, it is not considered that sufficient isocyanate groups for reacting with polyethylene-vinyl acetate copolymer with hydroxyl groups remain at thermal image transfer.
As disclosed in Japanese Laid-Open Patent Application 63-212588, when a micro-capsuled isocyanate and a thermoplastic polyol are contained in a surface portion of an image receiving sheet, the microcapsules will be ruptured by the pressure applied by a platen roller during printing, so that the crosslinking of the thermoplastic polyol takes place on the surface of the image receiving sheet. Thus, there is the risk that no components from a thermosensitive image transfer ink sheet are fixed to the image receiving sheet, without forming any clear images.
Furthermore, Japanese Patent Publication 60-59159 discloses a method of controlling the softening point of a constituent resin in an ink layer of a thermosensitive image transfer ink sheet by using an epoxy resin with a softening point of 60.degree. C. to 110.degree. C. and a coloring agent in combination. However, images printed on an image receiving sheet come off when rubbed at a temperature higher than 100.degree. C.